In recent years, energy saving has been strongly demanded from the viewpoint of prevention of global warming, and energy saving for domestic electric appliances has become an urgent problem. In particular, heat insulators having an excellent adiabatic performance (heat insulating efficiency) are demanded for thermally insulated equipments such as refrigerators, freezers, automatic vending machines, etc. from the viewpoint of efficient use of heat.
As general heat insulators, fiber materials such as glass wool, etc. and foam such as urethane foam, etc. are used. In order to enhance these heat insulators in adiabatic property, it is necessary to increase the heat insulator in thickness. Since there is a limitation to a space, into which heat insulators can be filled, however, such measures cannot be applied in case of the necessity for space saving and effective spatial use.
Attention is paid to vacuum heat insulators as heat insulators having a high adiabatic performance. Vacuum heat insulators are ones, in which a core is covered by an exterior covering having a gas-barrier quality, an interior of the exterior covering is reduced in pressure, and an opening of the exterior covering is fused.
Conventional vacuum heat insulators include one, in which an aggregate of inorganic fibers such as glass wool, etc. is cured by a binding agent to be used for a core. Such vacuum heat insulators are described in, for example, U.S. Patent publication No. 4,726,974 and Japanese Patent Unexamined Publication No. H8-28776. Since an aggregate of inorganic fibers is cured by means of a binding agent, the vacuum heat insulator has a sufficient strength and a sufficient planarity to be excellent in handling quality. However, such a vacuum heat insulator has the adiabatic performance (thermal conductivity) of about 0.007 W/mK at the degree of vacuum of 1.33 Pa, the adiabatic performance is same as that of a vacuum heat insulator in which powder filling is used as a core. Thus it is demanded to enhance the adiabatic performance beyond such adiabatic performance.